Microsoft has experience handling online code repositories and team services. Azure integration has the potential to improve deployment pipelines for many users. Microsoft has lots of experience doing things at scale, and can potentially find cost reduction maneuvers for Github. Visual Studio integration can bring more users to Github. Cooperate users are more likely to upgrade their existing agreement with Microsoft than they are to enter a finical relationship with a new company. Microsoft has just as much or more to offer Github as any other top 5 tech company.
https://blogs.msdn.microsoft.com/devops/2018/03/15/gvfs-for-...
Well, the world has changed and there is no longer any opportunity for a 90s Microsoft. So I'm pretty sure that unless the world changes back to the 90s, Microsoft wouldn't survive such a turn.
If they've managed to change that much, then nothing prevents them to make the opposite turn and return to the monopolistic Microsoft as soon as a new CEO steps in.
That's why I prefer Microsoft to stay away from the products and projects everybody loves. Furthermore, I don't think Microsoft has anything to contribute to GitHub.